The present invention relates to RF transmission systems and methods, and more particularly, to peak-to-average power ratio (PAR) reduction in orthogonal frequency division multiplexing (OFDM) systems with phase shift keying (PSK) input signals using blind selected mapping (SLM).
OFDM is a transmission format that has gained much popularity over the past decade. It is used in Asymmetric Digital Subscriber Line (ADSL), Wireless Local Area Network (WLAN), European Digital Audio Broadcast (DAB), Digital Video Broadcast (DVB) applications, and is under consideration for Worldwide Interoperability for Microwave Access (WiMAX).
A serious drawback of OFDM is the high peak-to-average power ratio (PAR), or crest factor (CF) (which is the square root of the PAR), of its time domain waveforms. The occurrence of occasional large peaks places stringent demands on the dynamic range and linearity of analog components such as the digital-to-analog converter (DAC) and the power amplifier (PA). Because the PA is a peak power limited device, a high PAR signal will have to be transmitted at a low average power level if nonlinear distortion is to be avoided. The resulting DC to RF power conversion efficiency is low, thus the battery drain or the network operating cost would be high. PAR reduction is often necessary before the signal is sent to the DAC and the PA.
Generally speaking, PAR reduction algorithms fall into two categories. Algorithms with distortion such as clipping and companding are relatively simple to implement, but the drawbacks include in-band distortion and out-of-band spectral regrowth. This is discussed by J. Tellado, in Multicarrier Modulation with Low PAR: Applications to DSL and Wireless, Kluwer Academic Publishers, 2000, and S. H. Han et al., “An overview of peak-to-average power ratio reduction techniques for multicarrier transmission,” IEEE Wireless Communications, vol. 12, no. 2, pp. 56-65, April 2005, and references cited therein. Distortionless PAR reduction algorithms include coding, selected mapping (SLM), partial transmit sequence, tone reservation, tone injection, active constellation extension, and so forth. Distortionless methods tend to be more computationally intensive, and result in lower data rate or spectral efficiency because of the need to transmit side information.
The SLM method, such as is discussed in papers by R. W. Bauml, et al., “Reducing the peak-to-average power ratio of multicarrier modulation by selected mapping,” Electronics Letters, vol. 32, no. 22, pp. 2056-2057, October 1996, and D. J. G. Mestdagh and P. M. P. Spruyt, “A method to reduce the probability of clipping in DMT-based transceivers,” IEEE Trans. on Communications, vol. 44, no. 10, pp. 1234-1238, October 1996, for example, has good PAR reducing capability and is distortionless. The drawback of the SLM method disclosed in these papers is that side information needs to be transmitted as well, thus lowering the data rate. It would be desirable to have an improved SLM technique that avoids the transmission of any side information and entails an accurate detection scheme to retrieve the side information at the receiver. The present invention addresses this issue when the frequency domain OFDM signal Sk is drawn from a constant modulus (i.e., PSK) constellation.
A concept related to the presently disclosed algorithm was discussed in Y. C. Cho, et al., “Selected mapping technique with novel phase sequences for PAPR reduction of an OFDM signal,” Proc. IEEE 60th Vehicular Technology Conference, vol. 7, pp. 4781-4785, September 2004. Denote by Xk(d) the dth alternative representation of the information signal Sk. In the Cho et al paper, up to 10% of the subcarriers whose positions are pre-determined according to a lookup table (LUT) indexed by d, are mapped from Sk to Xk(d) according to Xk(d)=−μSk, μ>1; the rest subcarriers remain unchanged; i.e., Xk(d)=Sk, μ>1 is a carefully chosen constant to ensure that “the minimum distance between modified signal point and adjacent signal point is larger than the minimum distance between the original signal points.” Cho, et al. reported performance degradation in terms of the PAR reducing capability as compared with the original SLM, as well as an increase in the average transmit power. Cho et al. limit the phase changes in Xk(d) to π (as in Xk(d)=−μSk) and 0 (as in Xk(d)=Sk) only and their corresponding probabilities are much skewed (less than 10% and more than 90%, respectively).